In the field of air pollution prevention attributable to the exhaust gas of internal combustion engines, particularly, for automotive use, it is recognized as important to precisely control the air-to-fuel ratio of a combustible mixture fed to the engines. A feedback control system as one of hitherto proposed techniques employs an exhaust sensor for developing a feedback signal representing a concentration of a certain component (which may be O.sub.2, CO, CO.sub.2, HC or NO.sub.x) of the engine exhaust gas as the indication of the air-to-fuel ratio realized in the engine. In a control circuit of this control system, the output of the exhaust sensor is compared with a reference signal which represents a desirably preset air-to-fuel ratio. Then the control circuit produces a control signal for controlling the operation of an air-fuel proportioning device such as a carburetor or a fuel injection system based on the magnitude of a deviation of the sensor output from the reference signal. The control signal is proportional to the deviation or represents the result of an integration of the deviation, but may comprise both a proportional component and an integral component. In response to this control signal, the fuel feed rate and/or the air feed rate in the air-fuel proportioning device is minutely regulated, along with the usual regulation according to variations in principal factors in the engine operation typified by the degree of opening of the throttle valve, in order to maintain the air/fuel ratio at a preset ratio. The value of the preset ratio is determined so that an exhaust gas treatment apparatus such as a thermal reactor or a catalytic converter included in the exhaust system may work at optimum efficiency. For example, the preset ratio is at or in the vicinity of the stoichiometric air/fuel ratio when a catalytic converter contains a "three-way" catalyst which can catalyze both the reduction of nitrogen oxides and the oxidation of carbon monoxide and hydrocarbons.
In general, currently available exhaust sensors have a considerable high internal impedance which varies as the temperature varies. At present, the most familiar exhaust sensor is an oxygen sensor which operates on the principle of a concentration cell and has as an essential component a layer of an oxygen ion conductive solid electrolyte typified by zirconia stabilized with calcia. The internal impedance of this type of oxygen sensor is on the order of 100 k.OMEGA. at about 500.degree. C. but rises to the order of 1 M.OMEGA. or above at a lower temperature of 200.degree.-300.degree. C.
To precisely detect the output of an exhaust sensor which exhibits a great variation in its internal impedance, an input circuit as part of the control circuit of the above described control system must have a very high impedance, on the order of 10 M.OMEGA..
In conventional control circuits, a familiar input circuit is constructed by the use of a transistor or a field-effect transistor as a principal element. The input impedance of this type of circuit is determined by the impedance characteristic of the employed transistor and, accordingly, can hardly be made above several megohms. It is difficult, therefore, to precisely detect the output of the exhaust sensor while the sensor is exposed to an exhaust gas stream of relatively low temperatures as experiences immediately after starting of the engine or during a continued idling of the engine. From this reason, the operation of the air/fuel ratio control system is usually interrupted while the exhaust gas temperature is not sufficiently high.